This application claims the benefit of Korean Patent Application No. 2001-45219 filed on Jul. 26, 2001, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens formed of a single lens and having a high NA (numerical aperture) which has an improved structure to overcome limitations of a conventional objective lens formed of a single lens, and an optical pickup apparatus adopting the same.
2. Description of the Related Art
To increase density in recording information on a recording medium, the size of a light spot formed on the recording medium to record and/or reproduce data needs to be reduced. The size of a light spot is proportional to a wavelength xcex of light in use, as shown in Mathematical Formula 1, and inversely proportional to the size of NA of an objective lens. Thus, as the NA of the objective lens increases, the recording density increases.
Mathematical Formula 1
Size of light spotxe2x88x9dxcex/NA 
However, due to limitations in manufacturing, it is difficult to provide an objective lens formed of a single lens having a high NA, for example, 0.7 or more, while satisfying an allowance condition of optical aberration (when slope angle less than 55xc2x0 and optical path difference (OPD)=0.03xcex rms, decenter greater than 4 xcexcm and tilt greater than 2 arcmin). This is because of a restricted material of a lens and because a spherical shape of the lens is designed such that it is inappropriate for injection. That is, an objective lens having a short focal length and a high NA and formed of a single lens has a large curvature of a spherical surface so that an angle of a tangent at an effective surface becomes 55xc2x0 or more. This slope angle makes the manufacturing of a mold difficult and decreases a degree of allowance in decentering between lens surfaces during manufacturing of a lens, thus lowering production yield.
Thus, an objective lens having a high NA and which is easy to manufacture and to secure allowance in assembly is needed. Also, as the NA increases, since an effect by wavefront aberration according to a thickness of a recording medium increases, the thickness of a recording medium, that is, the thickness of a protective layer to protect a recording surface, is required to be thin.
As is well known in the field to which the present invention pertains, DVDs (digital versatile disc) have a protective layer having a thickness of 0.6 mm to protect a recording surface of a recording medium. When a protective layer is thick, as in a DVD, a working distance (WD) between an objective lens and a recording medium must be sufficient. Accordingly, the thickness of a protective layer restricts an increase of NA of the objective lens needed to miniaturize the objective lens formed of a single lens and to provide high density recording. In order to manufacture an objective lens having a small diameter and a sufficient WD to prevent the objective lens from contacting a recording medium, a curvature of a spherical surface increases while allowances in eccentricity and angle of view between surfaces of the lens decreases, so that productivity deteriorates.
An objective lens formed of two lenses, as illustrated in FIG. 1, has been suggested by Japanese Patent Publication No. 10-123410 to satisfy a need for the thickness of a thin protective layer and overcome the limitations on the objective lens formed of a single lens.
Referring to FIG. 1, a conventional objective lens 10 includes a first lens 11 to condense incident light and a second lens 13 disposed between the first lens 11 and the recording medium 1 to increase an NA of the objective lens 10. When a red wavelength, for example, light having a wavelength of 635 nm, is used to secure assembly allowance, curvature is assigned to four divided lens surfaces so that a high NA of 0.7 or more can be achieved.
The objective lens 10 includes at least one aspheric surface and uses a low dispersion material to reduce chromatism. An NA of 0.7 or more can be achieved by using a low dispersion material exhibiting an Abbe""s number of 40 or more in line d and an NA of 0.8 or more can be achieved by using a low dispersion material exhibiting an Abbe""s number of 60 or more.
Given that the focal length of the first lens 11 disposed at a position where light is incident is f1 and the synthesized focal length of two lenses 11 and 13, that is, the overall focal length of the objective lens 10, is f, the objective lens 10 provides that 1.7 less than f1/f less than 2.5.
Also, when a thickness T of a transparent protective layer to protect a recording surface with respect to NA satisfies conditions of Inequalities 2 below, the objective lens 10 enables correction of aberration.
Inequalities 2
When 0.7 less than NA less than 0.8, Txe2x89xa60.32 mm; 
When 0.8xe2x89xa6NAxe2x89xa60.9, Txe2x89xa60.20 mm; and 
When 0.9xe2x89xa6NA, Txe2x89xa60.11 mm. 
Also, when a refractive index of a lens having a relatively large curvature of a lens surface of two lenses 11 and 13 is n1 and a reflective index of a lens having a relatively small curvature is n2, the objective lens 10 satisfies n1 greater than n2.
Also, when the diameter and the working distance of an incident light beam are BW and WDxe2x80x2, respectively, the objective lens 10 provides that 1.0xe2x89xa6BW less than 4.5 and 0.05xe2x89xa6WD. The BW and WDxe2x80x2 satisfy conditions of Inequalities 3 below with respect to the NA of the objective lens 10.
Inequalities 3
When 0.7 less than NA less than 0.8, WDxe2x80x2xe2x89xa60.25676BW+0.039189; 
When 0.8 less than NA less than 0.9, WDxe2x80x2xe2x89xa60.14054BWxe2x88x920.064865; and 
When 0.9 less than NA, WDxe2x80x2xe2x89xa60.096429BWxe2x88x920.244640. 
The conventional objective lens 10 can realize a high NA of 0.8 or more with respect to a recording medium having a protective layer thinner than that of a DVD. Also, the objective lens 10, which is formed of two lenses 11 and 13, can satisfy the condition of a thin protective layer and simultaneously overcome the limitation on the objective lens formed of a single lens.
However, the conventional objective lens 10 formed of two lenses needs a holder to maintain a distance between the two lenses 11 and 13, that is, an air gap, to be constant. Thus, since the conventional objective lens 10 has a high NA, allowances in distance and angle between the two lenses 11 and 13 are too small during assembly of the holder, so that production yield is deteriorated. Furthermore, since an actuator to control focus by driving the two lenses 11 and 13 being integrated is needed, a recording/reproducing apparatus is complicated and large.
Accordingly, it is an object of the present invention to provide an objective lens having a high NA and formed of a single lens which has an improved structure so that the limitations on the conventional objective lens formed of a single lens and the problems of the conventional objective lens formed of two lenses are overcome, and an optical pickup apparatus adopting the objective lens.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing and other objects of the present invention are achieved by providing an objective lens formed of a single lens, wherein a working distance of the lens is less than a thickness at the center of the lens.
The foregoing and other objects of the present invention may also be achieved by providing an objective lens formed of a single lens, wherein a thickness at the center of the lens is 0.8 times or more an effective diameter of the lens.
The foregoing and other objects of the present invention may also be achieved by providing an optical pickup apparatus comprising a light source to generate and emit a light beam having a predetermined wavelength, an objective lens to focus the light beam emitted from the light source to form a light spot on a recording surface of a recording medium, and a photodetector to receive the light beam reflected by the recording surface of the recording medium, wherein the objective lens is formed of a single lens and a working distance of the objective lens is less than a thickness at the center of the objective lens.
The foregoing and other objects of the present invention may also be achieved by providing an optical pickup apparatus comprising a light source to generate and emit a light beam having a predetermined wavelength, an objective lens to focus the light beam emitted from the light source to form a light spot on a recording surface of a recording medium, and a photodetector to receive the light beam reflected by the recording surface of the recording medium, wherein the objective lens is formed of a single lens and a thickness at the center of the objective lens is 0.8 times or more than an effective diameter of the objective lens.
An embodiment of the present invention provides that when an NA of the objective lens is 0.8 or more, a working distance of the objective lens satisfies the following Inequality:
WDxe2x89xa6xcex1BWxe2x88x920.2T, 
wherein WD is a working distance, BW is a diameter of an incident light beam, T is a thickness at the center of the objective lens, xcex1=1/(2 tan xcex8), and xcex8=Arcsin(NA).
An embodiment of the present invention also provides that the recording medium has a structure adopting a first reflection surface recording method, and that a protective layer of the recording medium has a thickness in a range of 40 nm-100 xcexcm.
An embodiment of the present invention also provides that the optical pickup apparatus further comprises a reflection type compound prism formed of a plurality of prisms to make an incident light beam emitted from the light source proceed by reducing the size of the light beam in a direction of height by using a difference in angle between surfaces of the prisms, and the light beam reflected by a surface forming an angle less than 45xc2x0 with respect to a horizontal plane which is perpendicular to the direction of height, so as to proceed toward the objective lens disposed in the direction of height.